The goal of this project is to elucidate the function of cyclin C in tumorigenesis. In the proposed work we will focus on the role for cyclin C in T-cell lymphoblastic leukemia (T-ALL, in Aims 1 and 2), while in Aim 3 we will extend our analyses to prostate and lung cancers. Cyclin C was cloned over 20 years ago, together with cyclins D1 and E, in a screen for mammalian and Drosophila cDNAs that can complement mutations in Saccharomyces cerevisiae CLN1-3 genes. Subsequently, cyclins D1 and E have been extensively studied, and their involvement in cancer has been extremely well documented. In contrast, the role of cyclin C in human cancer is much less understood. In order to analyze the function of cyclin C in tumorigenesis, we generated conditional cyclin C knockout mice, using gene targeting in embryonic stem cells. Unexpectedly, we found that an acute ablation of cyclin C in hematopoietic cells led to a strong upregulation of intracellular Notch1 (ICN1) levels. We observed that cyclin C-CDK8 kinase phosphorylates ICN1, which in turn allows binding of ICN1 to an F-box protein Fbw7, and triggers ICN1 poly-ubiquitination and subsequent proteosomal degradation. Ablation of cyclin C disrupts this process, leading to strong upregulation of ICN1 levels. Analyses of conditional cyclin C knockout mice revealed that these animals display increased susceptibility to T-ALL. Importantly, cyclin C heterozygotes also display increased tumor susceptibility. Moreover, we observed that the cyclin C gene is heterozygously deleted in a substantial fraction of human T-ALL, and these tumors display reduced cyclin C levels. Collectively, these observations suggest that cyclin C likely functions in vivo as a haploinsufficient tumor-suppressor. In the proposed work we will extend these observations. In Specific Aim 1, we will study the exact molecular function of cyclin C in T-ALL using human tumor cell lines. In Specific Aim 2, we will utilize primary, patient-derived human T-ALL samples to search for mutations or deletions in genes encoding cyclin C kinase partners (cyclin-dependent kinases CDK8, CDK19 and CDK3). In Aim 3, we will extend our studies to prostate and lung cancers.